Tubular packed bed catalytic reactors are well known in the art for numerous chemical reaction processes. In general, a reactor tube is filled with a particulate catalyst and chemical reactants are flowed through the tube where they undergo a chemical reaction. The chemical reactants are usually in a gaseous form, but in some cases may be liquid, and the same applies to the products of the reaction. In most cases, heat is either generated or consumed by the reaction, which itself may require elevated temperatures to achieve practical reaction rates.
Numerous criteria influence the design of a catalytic reactor. Among the typical considerations are: (1) the reaction rate and corresponding amount (volume) of catalyst needed per unit of reactant flow; (2) the heat and temperature requirements for the reaction; and (3) the fluid flow and pressure requirements on the inside of the tubes.
Some of the typical design implications and tradeoffs for reactor geometry, particularly tube length and diameter are as follows. Relatively small diameter tubes provide better heat transfer characteristics since they have a higher external surface to internal volume ratio. However, small diameters restrict flow, requiring higher inlet pressure. They also require longer lengths of tubes for a given catalyst volume due to smaller volume per unit length. On the other hand, relatively large diameter tubes provide less resistance to flow, requiring a shorter length for the same catalyst volume. However, tubes with larger diameters generally have poor heat transfer characteristics due to a relatively lower external surface to internal volume ratio.
The balance between these factors will ultimately lead to a design decision where a given catalyst volume is packed into a tube of a given diameter and length. In order to manage pressure drop in the catalytic reactor assembly to a practical level, it is typically favored to arrange a number of tubes in parallel, rather than a single, long tube. Such tube bundles are commonly encountered across a wide array of applications.
In the field of relatively small scale reformer systems, additional constraints are imposed upon the design. Typically, the catalytic reactor assembly must be confined to a small external volume, while maintaining good temperature and heat transfer characteristics. The cost of the system can be an overriding factor in the design, and designs that minimize fabrication steps are therefore favored—so minimizing the number of tubes is favored for cost reasons. These additional constraints may be at odds in some cases. For example, a design might be feasible with a single long length of tube of a given diameter, but for space constraints, this design would be discarded in favor of a tube bundle, with higher fabrication costs.
On top of these high level design considerations, other practical matters need to be taken into account. In the case of a single or bundle of straight tubes, orientation of the tubes can be significant for long term performance stability. This is due to processes of catalyst particle attrition and settling that can occur slowly over time and may be accelerated by external factors such as vibration. The result of these aging processes is a reduction in the volume occupied by the catalyst over time, and the resulting empty volume in the tubes can allow the reactant flow to bypass the catalyst in the case of horizontal orientation. In the case of vertical orientation, catalyst settling can lead to a high pressure drop developing at the bottom of the tube, where the fine particles will tend to collect. The corresponding empty volume at the top of the tube can lead to potential problems since the empty volume will have different heat characteristics from the packed tube and may, in instances where external heat is applied, lead to local overheating and accelerated tube failure. In large scale installations, these problems are usually managed by appropriate maintenance schedules and procedures on the catalyst bed. In small scale systems, however, regular maintenance on the catalytic bed is generally not practical, instead requiring replacement of the entire catalytic reactor assembly when performance has degraded to an unacceptable level.
For reactor designs having multiple tubes operated in parallel, consideration must be given to equalizing reactant flow between the multiple tubes and maintaining the flow equal during operation. For reactions involving an increase in the number of moles from the reactants to products, the potential for aggravated flow mal-distribution exists since a relatively underperforming or “dead” tube will provide a path of lower resistance for flow of reactants, which will thereby remain unconverted. A dead tube might result from a degraded, lower activity catalyst or from relatively poor heat transfer in relation to other tubes, resulting in a cold tube or tubes with lower catalyst activity.
For incorporating a catalytic reformer assembly in a system to, e.g., produce hydrogen by steam reforming of methanol (methyl alcohol, or CH3OH), consideration must be given to providing the required heat input into the reformer assembly both for maintaining the temperature of the reformer and to provide the necessary heat of reaction. This heat may be provided by a burner for example. As it is advantageous to provide equalized heat input to the reformer tubes, the burner design and tube arrangement are mutually dependent. Again, when multiple tubes are operated in parallel, the heat input and concomitant burner design become significant in order to avoid the occurrence of dead tubes as described above.